1. Field of the Invention
The present invention relates to a mechanism for controlling displacement of a hydrostatic transmission system of a vehicle, that is, a linkage between respective displacement-control devices (such as movable swash plates) of a hydrostatic stepless transmission for traveling (hereinafter referred to as a traveling HST) and a hydrostatic stepless transmission for steering (hereinafter referred to as a steering HST) installed on a vehicle.
2. Related Art
Conventionally, as disclosed in Japanese Laid Open Gazette No. Hei 10-95360, there is a well-known vehicle equipped with a traveling HST and a steering HST, wherein, while the output power of the traveling HST, whose direction can be switched between directions for forward traveling and for backward traveling, is transmitted to a pair of left and right axles, the output power of the steering HST, whose direction can be switched between directions for left turning and for right turning, is split into opposite rotational powers and the two powers are transmitted to the respective left and right axles so as to differentially rotate the left and right axles for making a turn of the vehicle.
The vehicle having the traveling HST and the steering HST especially requires the linkage between displacement-control devices of both the HSTs to be a reverse logic linkage, such as to coincide the steering operational direction (e.g., the turning direction of a steering wheel) with the actual left or right turning direction of the vehicle regardless of whether the traveling HST is set for forward traveling or backward traveling, thereby preventing wrong steering operation. For example, due to the reverse logic linkage, whether the vehicle travels forward or backward, the vehicle turns left by leftward rotation of a steering wheel. The vehicle may also or alternatively require the linkage between displacement-control devices of both the HSTs to be designed to reduce the output speed of the traveling HST in correspondence to the steering operational degree (e.g., the turning angle of a steering wheel from the straight traveling position), thereby preventing a high-speed sharp (or spin) turn.
In the prior art disclosed in the above-mentioned document, a conic mechanical linkage has an output section connected to servo-actuators for respective movable swash plates of the HSTs. The conic mechanical linkage has an input section connected to a forward/backward traveling direction selection lever for selecting the output direction of the traveling HST, and to a steering wheel for the output speed and direction of the steering HST. The conic mechanical linkage realizes the reverse logic linkage. The conic mechanical linkage also realizes reduction of traveling speed in correspondence to an increase of the turning angle of the steering wheel. However, the conic mechanical linkage is quite complicated because it requires three-dimensional angle adjustment among component parts or for other reasons.
It may be suggested that a hydraulic, pneumatic or electric actuator can replace the complicated mechanical linkage. However, if the vehicle is a small-size vehicle having no hydraulic pressure source but that for the HSTs, the hydraulic actuator is unreasonable in cost. Further, the pneumatic actuator requiring intake air of an engine, and the electric actuator are disadvantageous in convenience.